1. Field of the Invention
The present invention generally relates to a battery residual capacity measuring system and a battery residual capacity measuring method for electric vehicles, and particularly, it relates to a battery residual capacity measuring system and a battery residual capacity measuring method for electric vehicles in which a defined current residual capacity of a battery is determined to be displayed in consideration of a characteristic of the battery even in a fully accelerated travelling of the vehicle.
2. Description of Relevant Art
Generally, the battery has a variety of discharge characteristics in dependence on a load and a temperature. Discharge current and terminal voltage of battery violently vary, as the load varies.
It therefore is not easy to define a residual capacity of a battery. The residual capacity of battery yet tends to substantially linearly descend with time. Typically, it thus has been determined, relying on this tendency.
For example, in electric vehicles in recent years, a conventional system for measuring a residual capacity of an installed battery samples data on a battery discharge current (hereafter simply "amperage") and data on a battery terminal voltage (hereafter simply "voltage") at intervals of a predetermined time during a travel, and calculates a correlation coefficient between sampled amperage data and sampled voltage data to have an approximate line determined therefrom to be mapped in a coordinate system defined by an amperage axis and a voltage axis, as the coefficient exhibits a significant negative or high correlation.
The measurement system then reads an intersect between the approximate line and a representative line of a preset reference amperage, as a point corresponding to a residual capacity to be displayed.
The vehicle travels along a road that may incline and decline.
At a steep incline, it may be fully accelerated along the distance. In such a fully accelerated travelling (hereafter sometimes "full acceleration" or "high load condition"), the battery tends to have amperage and voltage values concentrated about a point without significant variations, as illustrated in FIG. 1.
Therefore, sampled data fail to have a significant correlation in the amperage vs. voltage coordinate system, nor to serve for the measurement system to determine a defined current residual capacity.
For a correlation to be significant, sampled data should be dispersed on a plane defined by amperage and voltage axes of the coordinate system, as illustrated in FIG. 2.
In the case both amperage and voltage values are concentrated as in FIG. 1, therefore, the conventional system fails to update a previous residual capacity, which is thus left, as it is displayed, until the system detects a subsequent significant correlation.
Accordingly, during a travel interval under full acceleration, the previous residual capacity is kept displayed, disabling the driver to know a defined current residual capacity.
In other words, over an interval of travel with high load, as an identical value to the previous value is displayed, the displayed residual capacity does not change despite an increasing travel distance, as illustrated in FIG. 3.
As a result, the displayed residual capacity has an errorneous value during full acceleration.